Multilevel metallization of large scale microelectronic devices has become an area of significant interest as devices are scaled down to increase circuit density. As device size is scaled down, metallization reliability is becoming increasingly significant, requiring good conductivity, electromigration resistance, and adherence to dielectric substrates. Copper has attracted significant attention due to its lower electrical resistivity and superior electromigration lifetime in comparison to widely-used aluminum based metals.
Nonetheless, technical problems exist regarding the use of copper in microelectronic devices. For example, the interface between copper (Cu) and an insulating or dielectric material, such as silicon dioxide (SiO.sub.2), can suffer from poor adhesion. Another Cu-dielectric interface problem is the undesirable transport of copper ions into the dielectric material. The copper ions can potentially disrupt the properties of the dielectric. Moreover, if transported through the dielectric to the electrically active Si, copper can cause additional problems.
The second interface, the exposed copper surface, is susceptible to oxidation during processing, forming copper-containing oxides. This can be particularly troublesome, since copper, unlike aluminum, on continued exposure will continue to react with oxygen, thus adversely impacting the resistivity of the copper layer and potentially causing other problems. As a result, adequate protection (passivation) of these interfaces must be achieved to obtain high quality, reliable devices.
To form a diffusion barrier/adhesion promoter layer, traditional methods have focused on placing a suitable material between the copper and the dielectric. Several materials have been employed, including refractory metals and carbon. However, these materials can suffer from various limitations and disadvantages. Thus, to the best of the inventors' knowledge, no one material has been widely accepted by the industry.
Regarding surface passivation, doping the copper with a metal which can be used to passivate the exposed copper surface has been attempted. More particularly, this method involves utilizing physical vapor deposition (PVD) to codeposit the metals as a film onto the surface of a dielectric substrate. Subsequently, the co-deposited film is annealed and exposed to an oxidizing ambient to form a passivating surface layer. As reported by Ding et al., J. Appl. Phys. Lett. 75 (7), 3627 (1994), and Ding et al., J. Appl. Phys. Lett. 64, 2897 (1994), aluminum and magnesium have been employed due to their ability to form extremely oxidation resistant surfaces.
Current trends in microelectronic device processing use metallization layers with higher aspect ratios, i.e., deeper trenches and vias, to increase circuit performance. PVD, however, has been shown to be largely ineffective in depositing metal onto surfaces with such dimensions.
One deposition technique which has been found to be useful is chemical vapor deposition (CVD). As opposed to PVD, CVD deposits metal onto a substrate with the aid of a gas-phase chemical reaction, thus allowing the trenches and vias to readily receive the material. Nonetheless, utilizing CVD for co-deposition of metals poses severe technical challenges. Accordingly, surface passivation by CVD co-deposition cannot be readily achieved.
Other recent passivation attempts have focused on simultaneously accomplishing surface passivation and diffusion barrier/adhesion promotion through the use of refractory metals and refractory metal alloys as reported by Lin et al., MRS Bulletin XVIII, 52 (1993). In this process, metals like titanium, chromium, molybdenum, and vanadium are deposited onto silicon dioxide substrates, with copper being subsequently deposited on top of this metal layer. The structure is annealed and exposed to an ammonia ambient such that the diffused refractory metal forms a nitride layer which passivates the copper. The remaining metal serves as a diffusion barrier/adhesion promoter.
Although formation of barrier layers can be achieved, employing refractory metals is highly disadvantageous. For example, extremely high annealing temperatures (i.e., 500-650.degree. C.) are generally required for successful diffusion and nitride layer formation. High manufacturing temperatures are undesirable, however, because of the adverse impact such temperatures can have on the integrity of the device. Indeed, the trend is towards decreasing processing temperatures. This is especially true for efforts employing polymeric material as a dielectric. Additionally, process conditions must be closely controlled to minimize excessive dissolution of refractory metal into the copper which causes a significant increase in copper resistivity. Moreover, because some of these materials have not been widely employed in microelectronic device processing, a substantial investment in new manufacturing equipment would likely be required if such processes were to be implemented on a large scale in the future.